Contamination Characteristics And Exposure Assessment Of P-phenylenediamine Antioxidants In Road Dust

N,N’-substituted p-phenylenediamine（PPD）is a class of antioxidants widely added to rubber tyres that can be released and accumulate in road dust as they wear out.6PPD-Q（6PPD-quinone）is an ozonation product of N-（1,3-dimethylbutyl）-N’-phenyl-p-phenylenediamine（6PPD）,which has recently come under scrutiny due to its high toxicity to silver salmon.It is of great concern because of its high toxicity to silver salmon.There are limited studies on the environmental behaviour of this contaminant,which introduces significant uncertainties in health risk assessment.In this paper,we first developed a method for the quantification of p-phenylenediamine antioxidants（PPDs）and 6PPD-Q in road dust samples based on high performance liquid chromatography-mass spectrometry（HPLC-MS/MS）,and based on this method,we investigated the contamination characteristics of six PPDs and 6PPD-Q in dust from main roads in 55 major cities across China,and further carried out human exposure assessment.The optimum mobile phases A and B were aqueous ammonium acetate 0.8%by mass（p H=3）and acetonitrile,respectively,and the optimum column temperature was 25°C.The baseline separation of all target substances was good and the peak shapes of the peaks were improved compared with those before optimization.The baseline separation of all the target substances was good and the peak shapes were improved compared to those before optimization.Subsequently,the extraction solvents and the amount of extraction solvents were optimised and the optimum extraction conditions were 10 m L acetone for three extractions.The limits of detection of the six PPDs and 6PPD-Q were in the range of 0.24～2.29 ng/g,and the spiked recoveries were86.61～96.88%.To investigate the environmental stock of PPDs and 6PPD-Q in road dust nationwide,the pollution levels and distribution characteristics of PPDs and 6PPD-Q in dust from main roads in 55major cities,including 4 municipalities directly under the central government,27 provincial capitals and 24 other cities,were analysed.The results showed that the concentrations of the seven compounds were from high to low:6PPD-Q,6PPD,DNPD,DPPD,77PD,CPPD and IPPD,with the concentration ranges of 6PPD-Q:3.00～348.71 ng/g;6PPD:0.74～626.16 ng/g;DNPD:1.93～380.85 ng/g,respectively;DPPD:<MDL～145.45 ng/g;77PD:0.41～16.47 ng/g;CPPD:<MDL～17.61 ng/g;IPPD:<MDL～32.13 ng/g.The contamination levels varied greatly between cities,and the fugacity levels of the antioxidants 6PPD,DNPD and DPPD in the road dust samples were significantly higher than those of other three types of PPDs.The results of the correlation between the concentrations of the compounds measured by the quantitative analysis and the meteorological parameters of the day of sampling,the average meteorological parameters of the month of sampling,the average meteorological parameters of the season and the average meteorological parameters of the year of sampling showed that 6PPD-Q was significantly positively correlated with the average meteorological parameters of the month of sampling for PM_2.5and PM₁₀（P≤0.001）,and correlated with NO₂but not significantly（P≤0.05）.In addition,this study was conducted for urban motor vehicle ownership and correlation analysis between motor vehicle ownership and p-phenylenediamine antioxidants,and the results showed that there was no significant correlation between motor vehicle ownership and the target compounds.Traffic volume,vehicle tyre type and environmental conditions have a significant influence on the concentration and proportion of PPDs and 6PPD-Q in dusty soils.In order to better understand the pollution levels and distribution characteristics of PPDs and 6PPD-Q in different traffic functional areas,47 dust samples from different traffic functional areas were collected in Dalian in this experiment,and a comparative analysis of the pollution characteristics of different areas was carried out.The results showed that the highest total concentrations ofΣPPDs（a general term for the six PPDs）and 6PPD-Q were found in the road dust in the highway service area,while the lowest total concentrations were found in the suburban road dust,which may be due to the low traffic flow in the suburban roads and the high braking behaviour of motor vehicles in the highway service area,resulting in a large number of tyre wear particles entering the dust.After PLS-DA analysis（Partial Least Squares Discriminant Analysis）,the main component in the dust from the motorway service area was 6PPD-Q,while the DNPD in the dust from the suburban roads was significantly higher than that from other functional areas,and there was a principal component difference between the two.In this study,the quantitative analysis of PPDs and 6PPD-Q in the residential areas of Dalian was carried out.The results show that,taking the main road dust as the main source medium ofΣPPDs and 6PPD-Q,we can see thatΣPPDs and 6PPD-Q are able to migrate along the pathway of"main road dust-residential road dust-residential square dust-green belt topsoil"with decreasing levels and detection rates.Human exposure to PPDs and 6PPD-Q in dust samples was assessed based on the quantitative results of PPDs and 6PPD-Q.The oral intake ofΣPPDs and 6PPD-Q was assessed to be much greater than the dermal absorption route,and the intake of bothΣPPDs and 6PPD-Q via road dust showed that main roads>residential roads>residential squares,with main road dust accounting for more than 70%of the intake,and the daily intake of children was higher than that of adults,indicating a higher risk of this compound for children.Due to the toxic effects of 6PPD-Q on aquatic organisms,a further assessment of the runoff potential of 6PPD-Q was carried out,and the results showed that the predicted concentrations of 6PPD-Q in river or lake water exceeded the newly revised LC₅₀concentrations of 6PPD-Q in the five monitored cities,which sounded an alarm about the ecological risks caused by 6PPD-Q,and therefore we should pay attention to the relationship between PPDs in dust and 6PPD-Q levels in dust should be taken seriously.This study is the first nationwide investigation to report the overall pollution levels of PPDs and 6PPD-Q and the city-based compositional differences.The functional area-based pollution characteristics of PPDs and 6PPD-Q give new insights into the toxic effects of 6PPD-Q on fish,which may pose human health risks and require special attention.